1. Field of the Invention
The present invention relates to a device for measuring the vertical distance between at least two elements. A possible application for the device of the invention is the measurement of the variations of the vertical distance between a position on a floating installation and a mobile element. Among the possible applications of the device may, for example, be the control of an anti-heave system for a floating installation, such as is used in oil drilling ships or in marine engineering operations.
2. Description of the Prior Art
The distance between two points movable with respect to each other may be measured by installing a physical connection therebetween. A stretched wire, chain, or rack is possible for short distances, measuring the variations of the length of this connection. This is an inexpensive solution, but it is difficult to use when the distance is great, because of the deformation of the wire or chain, or when the installation of a physical connection hinders the operation of the mobile elements and restricts their freedom of movement.
Distance measurements may also be made without a physical connection by means of telemeters. Generally, these are discontinuous measurements made at time intervals spaced more or less apart depending on the rate of variation of the distances to be measured, by means of acoustic pulses.
Acoustic telemetry, which consists in measuring the propagation time of acoustic waves over the distance in question, is used in numerous fields: positioning at sea with respect to immersed points, measurement of the level of liquids and particularly that of the height of water under a ship by means of sonars etc, or in photography for adjusting the focussing of picture taking apparatus.
The distances may also be measured by means of electromagnetic waves. The devices used are radar or radio-altimeters. In the field of optical frequencies, laser telemeters are also known which provide a very high accuracy.
The above mentioned devices are generally poorly adapted in all cases where it is desired to determine a vertical distance between two points which are not on the same vertical and are driven with complex movements with respect to each other.
This is the case more particularly on floating installations where an anti-heave system maintains, at a substantially constant altitude with respect to the bottom of the mass of water, a crown block supporting, for example drilling equipment. In order to be able to control the correct operation of the anti-heave system, it is necessary to accurately know the variations of altitude of the stabilized crown block and also the variations of the vertical distance which separates it from any point related to the floating installation.
In French patent application EN 88/02.501 a device is described for measuring the altitude variations of a floating installation. The vertical distance between the stabilized crown block and the reference point on the floating installation may be calculated from direct measurements between these two positions using a telemeter of any type. But this is only possible if the telemetric measurements are corrected by taking into account the attitude variations of the floating installation determined by a device of known type for measuring the angular deviations with respect to the vertical. The assembly may deliver accurate measurements but its cost is generally very high.
The principle of measuring a vertical deviation between two points, by measuring the differential pressure at both ends of a flexible pipe filled with fluid and disposed between these points, has already been used. The problem to be solved is to obtain good accuracy in the pressure measurements and good compensation of the variations of the static pressure of the fluid under the action of temperature variations.